Noise in Cycles Interior Scene
Interior scenes to start with are very tough on Cycles, because there is so much mesh for Light Paths to bounce off of. For example, when I first tried to render Andrew Price's Subway in Cycles with no materials, I got this at 5,000 samples:



The fact is, interiors are tough on Cycles, but you can consider these things:


Only have mesh for what's in the frame. The more background behind the camera, the higher quality your renders will be.
256 samples is way too low to get any half decent results (except special cases). I render most everything at 500 samples minimum, 2,000 max.
Want a cheap and dirty way to reduce noise? Reduce your max bounces to 0, 1, possibly 2 and your min bounces to 0.
I know you want white, so make sure to have a Diffuse BSDF shader on everything, not just nothing. Materials will render better, though, if you want to lose them. This was my result of the same subway with materials:




Hope this helps and happy Blending!